Moisture measuring apparatus commonly known as tensiometers are used to measure the moisture content of soil and other like medium, and have been in use for some time. Tensiometers are commonly used by farmers and others responsible for managing soil irrigation, to monitor the moisture content of the soil and determine when it is necessary to water the soil and how much water should be applied. Using a tensiometer in this fashion a farmer can control the irrigation schedule for the soil on an ongoing basis, in conjunction with changes in weather and season, in order to optimize plant growth. Tensiometers are also commonly used in the scientific study of soils and plants.
A tensiometer generally includes a measurement tube whose interior provides a sealable measurement chamber, a porous tip attached to the bottom end of the tube, and a vacuum gauge connected to the chamber at the top end of the tube. The chamber is first filled with water and then sealed. The porous tip is then buried in the soil, thus establishing liquid contact between the water in the chamber and films of moisture in the soil surrounding the porous tip. Via the general process of capillary action, relatively dry soil tends to pull water from the chamber through the porous tip into the soil. However, since the chamber is sealed, only a small amount of water is pulled into the soil. The pulling effect of the dryer soil creates a negative pressure and related vacuum in the chamber, which can be measured on the gauge. As water is pulled out of the soil surrounding the tensiometer by evaporation and plants in the soil, more water is pulled from the chamber into the soil. Soil with relatively high moisture content, such as after rain or watering, pulls less water from the chamber, resulting in a lower negative pressure and related vacuum in the chamber, and a correspondingly lower reading on the gauge.
As stated previously, a small amount of water in the tensiometer's measurement chamber is lost over a period of time. Thus, air tends to accumulate in the chamber. The air may also initially be present, in dissolved form, in the water inside the chamber. The relatively low pressure in the chamber allows the air to come out of solution, resulting in the air gradually accumulating both on the chamber wall and at the top of the chamber. Air may also enter the chamber from the surrounding soil by diffusion through the porous tip. This accumulation of air in the chamber results in a reduction in the tensiometer's measurement sensitivity to changes in the moisture content of the soil. The end result is a degradation of the tensiometer's measurement accuracy. In order to address this problem and maintain the tensiometer's measurement sensitivity and accuracy, the tensiometer must be routinely serviced by adding water to the chamber to remove the accumulated air.
In some types of conventional tensiometers the accumulated air is removed from the measurement chamber by first manually removing a seal on the top of the chamber, then manually pouring more water into the chamber to refill it with water, and then manually installing the seal back onto the chamber. In addition to the inconvenience of this method, it is disadvantageous for a number of other reasons. For example, the aforementioned water refill operation is required to be performed routinely in order to maintain the tensiometer's measurement sensitivity and accuracy. However, during the period of time that the seal is removed, the vacuum in the chamber is lost, resulting in water flowing out of the chamber through the porous tip and into the surrounding soil. A long time may be required to pass before this local accumulation of water in the soil surrounding the tip disperses enough to allow the tensiometer to perform an accurate reading of the overall moisture content in the soil. Furthermore, the act of manually pouring more water into the chamber to refill it may not dislodge all the air bubbles that have accumulated on the wall of the chamber, especially for smaller diameter chambers. Hence, even after the chamber has been refilled with water, some air can be left in the chamber. Finally, installing the seal back onto the chamber asserts a positive pressure on the water in the chamber, which causes even more water to flow out of the chamber into the soil and further degrades the tensiometer's measurement sensitivity and accuracy.
Other types of conventional tensiometers contain a water reservoir connected to the measurement chamber through a valve which can be manually operated to allow water to flow into the chamber to refill it. However, these types of tensiometers generally also possess the aforementioned disadvantages in varying degrees.